Why you might not want ethanol in your gas tank

For many drivers and especially owners of small engines, boats, and older vehicles, ethanol-blended gasoline can mean lower fuel economy, greater risk of moisture-related problems, and potential material damage—reasons you might prefer to avoid it depending on how and what you drive. Modern cars generally handle E10 and even E15 without issue, but ethanol’s chemistry brings trade-offs that matter in certain vehicles, climates, and storage situations.

What ethanol in gasoline actually does

Ethanol is an alcohol blended into gasoline primarily as an octane booster and oxygenate. Common blends are E10 (up to 10% ethanol) nationwide and, increasingly, E15 (up to 15%) for 2001-and-newer light-duty vehicles approved by the U.S. Environmental Protection Agency. Flex-fuel vehicles are engineered to use E85 (51–83% ethanol), taking advantage of ethanol’s high octane, while most small engines and many marine applications are not designed for high-ethanol blends.

The main drawbacks: why ethanol can be a problem

These are the most common technical and practical reasons ethanol-blended gasoline can be undesirable in some tanks and use cases:

• Lower energy content, lower MPG: Ethanol contains less energy per gallon than pure gasoline. E10 typically reduces fuel economy by about 1–3% versus ethanol-free gas; E15 can trim roughly 2–5%. With E85, range drops far more in engines not optimized for it.
• Water attraction and phase separation: Ethanol is hygroscopic—it absorbs moisture from air. In vented or rarely used tanks, water can accumulate until the ethanol-water mix separates from gasoline, sinking to the bottom and causing hard starts, stalling, or corrosion.
• Corrosion and material compatibility: Water-ethanol mixtures can corrode metals like aluminum, zinc, and brass, and can degrade older rubber hoses, seals, and gaskets not rated for alcohol. Carbureted and pre-1990s fuel systems are most at risk.
• Storage sensitivity: For seasonal equipment or infrequently driven vehicles, ethanol blends break down faster, can encourage gum formation, and may loosen tank deposits that clog filters after refueling with E10/E15.
• Cold-start challenges at higher blends: While E10 is generally fine, higher ethanol content (especially E85) can complicate cold starts due to ethanol’s properties, unless the engine and calibration are designed for it.
• Potential warranty limitations: Although EPA permits E15 in 2001+ light-duty vehicles, not every manufacturer warranties E15 use across all model years; owners should confirm their specific vehicle’s guidance.
• Marine and small-engine risks: Boats and small engines often have vented tanks and sit unused, amplifying moisture uptake and phase separation. Carb jets and small passages are especially vulnerable to ethanol-related deposits and corrosion.

Taken together, these factors make ethanol a poor fit for equipment that sits for long periods, operates in humid environments, or uses older materials—situations where reliability and corrosion control matter more than octane or short-term cost.

Who should avoid ethanol blends

While E10 is ubiquitous and generally safe for modern cars, certain owners are better off seeking ethanol-free gasoline (often labeled “E0,” “REC 90,” or “non-ethanol”) or strictly following manufacturer limits:

• Boats and marine engines, especially in humid or coastal areas with vented fuel systems.
• Small engines: lawn mowers, chainsaws, string trimmers, snowblowers, generators, and pressure washers.
• Classic cars and older motorcycles with carburetors and legacy fuel-system materials (pre-1990s designs are most vulnerable).
• Vehicles and equipment stored for months at a time, including collector cars and standby generators.
• Engines with fiberglass or older plastic fuel tanks that can be softened by ethanol.
• Aircraft piston engines (avgas is specified without ethanol unless specifically approved by the aircraft/engine authority).

In these cases, avoiding ethanol or minimizing ethanol content reduces the chance of phase separation, corrosion, and hard starting after storage—helping preserve reliability and components.

When ethanol is fine—or even beneficial

For most 2001-and-newer passenger vehicles, E10 is routine and E15 is legally permitted by EPA; many late-model cars are calibrated to run on these blends. Ethanol raises octane, which helps modern high-compression or turbocharged engines resist knock, and it can reduce certain tailpipe pollutants like carbon monoxide and aromatics. Flex-fuel vehicles, and performance engines tuned accordingly, can exploit ethanol’s high octane to make more power—accepting the trade-off of lower miles per gallon.

What to expect for fuel economy by blend

Drivers often want a simple expectation for mileage differences when switching fuels. These ranges reflect typical outcomes in non-optimized engines:

• E10: roughly 1–3% fewer miles per gallon versus ethanol-free gas (E0).
• E15: roughly 2–5% fewer miles per gallon versus E0.
• E85: roughly 25–30% fewer miles per gallon in non-optimized engines; 15–25% fewer if the vehicle is flex-fuel or tuned to take advantage of ethanol’s octane.

Actual results vary with driving style, temperature, engine calibration, and fuel quality. If cost per mile matters most, compare pump price to expected MPG, not just the per-gallon price.

How to minimize problems if ethanol is unavoidable

If your local pumps mostly carry ethanol-blended gasoline, a few habits can reduce risk—especially for equipment that sits:

1. Check the owner’s manual: Confirm your vehicle or tool’s maximum approved ethanol content (E10, E15, or E85 for flex-fuel).
2. Use the right blend: Avoid E15 in non-approved engines; seek E0 or “REC” fuel for small engines and boats when possible.
3. Stabilize and store smartly: Add a fuel stabilizer rated for ethanol blends, run the engine briefly to circulate, and store with a near-full tank to limit humid air space.
4. Control moisture: Keep caps tight, avoid long storage in high humidity, and use water-separating marine filters where appropriate.
5. Maintain materials: Replace old rubber hoses, seals, and carb parts with ethanol-rated components; inspect filters after switching fuels.
6. Buy fresh fuel: Use busy stations to reduce the chance of stale gasoline and ensure proper turnover.
7. Monitor performance: Track MPG and watch for hard starts or a check-engine light after a blend change.

These steps won’t change ethanol’s chemistry, but they substantially reduce the likelihood of phase separation, corrosion, and varnish-related issues over time.

Policy and availability: why you’re seeing more E15

Federal renewable-fuel policy has encouraged ethanol blending for years, making E10 the default in most of the United States. In 2024 the EPA finalized rules enabling year-round E15 sales beginning in 2025 for several Midwestern states, including Illinois, Iowa, Minnesota, Missouri, Nebraska, Ohio, South Dakota, and Wisconsin; elsewhere, summer sales of E15 are still shaped by seasonal volatility rules and periodic waivers. At the pump, E10 is usually unlabeled or marked “up to 10% ethanol,” while E15 dispensers carry prominent labels. Ethanol-free gasoline remains available in many areas—often sold as “recreational” or “marine” fuel—though at a price premium.

The bottom line

You might not want ethanol in your gas tank if you own small engines, boats, older or stored vehicles, or if maximizing MPG matters more than the per-gallon price. For most modern daily drivers, E10—and increasingly E15—works as intended, with a modest fuel-economy trade-off. Choosing the right blend for your engine, storage habits, and climate is the surest way to avoid ethanol-related headaches.

Summary

Ethanol boosts octane and is safe for most newer cars, but it attracts water, can separate and corrode components, reduces MPG, and is hard on small engines, marine systems, and older fuel systems—especially in humid climates and during long storage. Use ethanol-free fuel where required, follow manufacturer limits, and apply stabilizers and good storage practices to mitigate risks.

What is wrong with ethanol in gas?

Ethanol is bad for some gas applications because its hygroscopic nature causes it to absorb water, leading to phase separation that can damage engines, fuel systems, and cause rust. It’s also a potent solvent, which can dissolve deposits and clog filters and carburetors, especially in older engines not designed for it. The production of corn-based ethanol has significant environmental costs, including increased greenhouse gas emissions and loss of natural habitat, often negating its intended benefits. 
For Engines and Fuel Systems

• Water Absorption: Ethanol is hygroscopic, meaning it attracts and absorbs water from the air. 
• Phase Separation: When ethanol absorbs enough water, the water and ethanol mixture can separate from the gasoline and settle at the bottom of the fuel tank. 
• Damage: This water-ethanol mixture is corrosive and can cause rust and damage to engine components, including metal parts and seals. 
• Clogged Filters: Ethanol’s solvent properties can loosen deposits in older fuel systems, leading to clogged fuel filters, injectors, and carburetors. 
• Corrosion: Ethanol can corrode certain metals commonly used in carburetors, such as zinc, magnesium, aluminum, and steel. 
• Engine Knock: Due to its molecular structure and high octane, ethanol can also cause uncontrolled combustion or “engine knock” if the engine’s compression ratio isn’t designed to handle it. 

For the Environment

• Land Use and Emissions: Growing corn for ethanol production requires large amounts of land, leading to deforestation and releasing carbon from soil, which can increase greenhouse gas emissions. 
• Increased Fossil Fuel Consumption: The farming, processing, and transportation of corn to make ethanol consume significant amounts of fossil fuels. 
• Negated Benefits: The carbon emissions from land use changes and the energy-intensive production process can negate any potential climate benefits of using biofuels like ethanol. 

Specific Issues with Older or Small Engines

• Material Incompatibility: Older engines and small power equipment often use rubber and plastic components that are not resistant to ethanol, leading to damage. 
• Accelerated Degradation: The combination of ethanol’s solvent action and potential water absorption can accelerate the deterioration of components in neglected fuel systems. 
• Fuel Decay: Ethanol contains a high amount of oxygen, which can cause gasoline to decay faster. 

Why do people not want ethanol in gas?

Ethanol is problematic mixed with gasoline because it is corrosive to many fuel system components and harmful to many elastomers long used in gasoline systems successfully. Ethanol has less energy per gallon than gasoline, so economy is degraded incrementally. IMO carbohydrates are food; hydrocarbons are fuel.

Is ethanol bad for a gas tank?

Ethanol blended gasoline can cause serious engine damage when used regularly. It is corrosive and highly water soluble, often leading to storage problems. If left, water in the fuel system can cause rusting, fuel degradation and other problems. Ethanol fuel is also associated with microbial contamination.

Do I want ethanol in my gas or not?

Most gases have a small percentage of ethanol which causes the fuel to burn hotter, this in turn causes more build up of carbon. Ethanol free relieves this issue and can be better all around for most engines and is recommended for engines that are older.